1. Field of the Invention
The invention relates to a process for preparing diorganopolysiloxanes which have an Si-bonded hydroxyl group only at one end of the molecule's chain.
2. Description of the Related Art
U.S. Pat. No. 3,445,426 A discloses the polymerization of hexaorganocyclotrisiloxane in the presence of catalytic amounts of a pentacoordinated silicon catalyst, an alkali metal, ammonium or phosphonium siliconate. The catalysts, however, have the disadvantage that they are costly and inconvenient to prepare and are very sensitive to moisture. Described as an initiator for the ring-opening polymerization is alcohol in combination with water, which always leads to polymer mixtures, i.e., to OH-terminated polymers and monoalkoxy-terminated polymers.
EP 0 331 753 A1 describes the polymerization of hexaorganocyclotrisiloxanes in the presence of sil(ox)anols and with the aid of alkali metal sil(ox)anolates as polymerization initiators. Alkali metal sil(ox)anolates likewise exhibit a sensitivity to moisture. In this case the catalysts used are organometallic compounds, which, as mentioned, possess an extreme sensitivity to moisture.
EP 0 338 577 A2 discloses the polymerization of hexamethylcyclotrisiloxane in the presence of trialkylsilanol and a lithium catalyst such as butyllithium. The use of organometallic compounds, however, is problematic from a safety standpoint.
L. M. Tartakovskaya et al., Vysokomol. Soedin. Ser. B 26 234, 1984 (Chemical Abstracts vol. 101, 73186d, 1984) describe the ring opening and polymerization of cyclic siloxanes in the presence of fluoride ions. On account of their toxicity, these catalysts are adjudged disadvantageous in industrial use.
DE 41 16 014 A1 describes the polymerization of cyclic siloxanes with catalysts comprising fluoride ions. These fluoride catalysts, however, have the disadvantage of side reactions with siloxanes, forming Si—F units. Moreover, the disclosed catalysts also exhibit toxicity, which makes industrial use more difficult.
EP 1 369 449 A1 discloses the polymerization of cyclic siloxanes with alcohols as initiator, with carbonate salts used as catalysts. The compounds obtained in this way, however, have the disadvantage of the formation of an alkoxy-silyl end group, which is not stable on storage. For example it may react with the silanol compound that is likewise present, with elimination of alcohol, and so may lead to chain extension.
The current processes therefore all have the disadvantage either of leading to unstable products or else of using reactants and/or catalysts that are problematic from a safety standpoint.